Cardano (ADA) was created after Bitcoin and Ethereum and thus could learn from their mistakes and build on their strengths. Its founder and CEO Charles Hoskinson call it a third-generation cryptocurrency, meaning that it already has some of the features that earlier cryptos are struggling to implement.
Cardano uses a modification of the Proof-of-Stake consensus algorithm called Oroboros, which makes the network extremely scalable. At the same time, Ethereum is only planning to transit from the clunky Proof-of-Work to the faster, more energy-efficient Proof-of-Stake. In this sense, ADA is already far ahead.
Yet another factor that is in play in the Ethereum vs Cardano race is team cohesion. ADA is a commercial project, and all the coders and researchers on the team are paid a salary. The Ethereum development community is a mishmash of teams, mostly unpaid, who argue with each other and propose competing ideas.